In general, waveguides and optical circuits are devices or systems of material boundaries, that are designed to confine, direct, and act on propagated electromagnetic waves, such as light or optical signals.
An electromagnetic wave is a vector field that has two primary and orthogonal polarization states or vector directions associated with its propagation. These polarization states are generally referred to as the Transverse Electric (TE) mode and Transverse Magnetic (TM) mode for optical waveguides. In the TM mode, the magnetic lines of flux are predominantly oriented parallel to the substrate on which the waveguide sits, while in the TE mode the electric lines of flux are predominantly oriented parallel to the substrate.
Waveguides and optical circuits are polarization dependent in general because the effective index of the waveguide or network of coupled waveguides are polarization dependent. That is, the waveguides and other optical devices and their performance are often sensitive to the polarization state of the electromagnetic wave propagation.
Many optical circuits have folded symmetry. That is, the circuit has a first half and a second half about which the circuit can be “folded” or “mirror imaged”. FIG. 1 represents a generic circuit previously known in the art which has a number of input ports, a number of output ports, and some plane of symmetry about which there is folded symmetry. The optical circuit can also have so called “point symmetry”. That is, the circuit of FIG. 1 can be folded along the symmetry plane and also flipped top-to-bottom (as if the second half of the circuit was reflected through a point rather than a plane). In such circuits the optical response has been made polarization independent by inserting a polarization rotator (or polarization converter) at precisely the symmetry plane as represented in FIG. 2. The effect of the modification represented in FIG. 2 is that a signal goes through one half of the circuit in one polarization (e.g., TE mode), gets rotated to the orthogonal polarization (e.g., TM mode), then goes through the second half of the circuit (which is symmetric to the first half) in the orthogonal polarization. Each polarization signal thus goes through one-half of the circuit in one polarization mode, and through the other half of the device in the orthogonal polarization mode, thus causing equalization of the signals. For example, one polarization rotator which has been used in a folded symmetry circuit comprises a non-integral component such as a half wave plate such as may be constructed from a polymer or from a birefringent material such as quartz, rutile, calcite, lithium niobate or YV04. Such wave plates must be incorporated into the optical circuit separately from the construction of the optical circuit itself, for example after a slot has been made in the substrate upon which the optical circuit is disposed. Examples of symmetric circuits include Mach-Zehnder (MZ) circuits and Arrayed Waveguide Gratings (AWGs).
In Mach-Zehnder circuits, a first waveguide path is split into two separate arms, which are then rejoined downstream, often after the signal in one arm has been subjected to an operation (FIG. 3). AWGs (for example as represented in FIG. 4) are commonly used as optical (de)multiplexers in wavelength division multiplexed (WDM) systems. These devices are capable of multiplexing a large number of wavelengths into a single optical fiber, thereby increasing the transmission capacity of optical networks considerably. The incoming light traverses a first free space and enters a bundle of optical fibers or channel waveguides. These fibers or waveguides have different lengths and thus apply a different phase shift to the signals passing therethrough. At the exit of the fibers or waveguides, the light traverses a second free space and interferes at the entries of the output waveguides which extend from the second free space in such a way that each output channel receives only light of a certain wavelength. Light entering the first free space is demultiplexed while in reverse is multiplexed.
Although polarization rotators at symmetry planes in symmetric circuits have been used in the past, such as polymer halfwave plates, as indicated above, no integrated optics polarization rotator has been previously known. It is an objective of the present invention to provide symmetric circuits constructed with integral rotators thereby providing monolithic solid state, robust polarization independent circuits.